Hypertension, or high blood pressure, is a common health problem; in North America, one in four adults has hypertension. Since hypertension is asymptomatic, irreversible cardiovascular complications may have arisen before the hypertension is even recognised.
One enzyme which plays a key physiological role in the regulation of blood pressure is angiotensin converting enzyme (ACE: peptidyldipeptide hydrolase, EC 3,4,15,1). ACE is involved in increasing blood pressure both through the renin-angiotensin system, by virtue of its ability to convert the inactive decapeptide angiotensin I to the vasoconstrictive and salt-retaining peptide angiotensin II (Skeggs et al., 1956) and through the kallikrein-kinin system, by virtue of its ability to inactivate the vasodilator and natriuretic nonapeptide, bradykinin (Yang et al., 1970).
Inhibition of ACE activity therefore provides a means of lowering blood pressure.
The first potent and specific inhibitors of ACE activity were discovered by Ferreira (1965), who showed that an extract of the venom of the Brazilian arrowhead viper, Bothrops jararaca, potentiated smooth muscle contraction, caused hypotension and increased the capillary permeability induced by bradykinin. The so-called ‘bradykinin potentiating factors’ (BPF's) were later isolated from a number of snake venoms and found to be short peptides which are potent ACE inhibitors.
A class of synthetic ACE inhibitors has also been made and marketed commercially, the first being captopril (D-2-methyl-3-mercaptopropanoyl-L-proline). Although many drugs of this group have been found to be invaluable for lowering high blood pressure, their prolonged use can be accompanied by unwanted side effects. There therefore remains a need for new therapeutic agents to control hypertension.
It has been known for some time that ACE inhibitory peptides can be obtained by proteolytic digestion of proteins from various sources, including fish (eg. EP 1094071), animal milk proteins (eg. WO 99/65326) and plants (eg. Kawakami et al., (1995); Yano et al., (1996); Pedroche et al., (2002); Wu et al., (2002)). Most of the methods described for obtaining ACE inhibitory peptides from plant proteins involve preliminary purification of the protein before hydrolysis, which adds to the cost and complexity of the process and therefore to the cost of the product.
There remains a need for improved processes for preparing ACE inhibitory compositions from plant materials and for further sources of ACE inhibitory peptides of high specific activity.